We study genes of the rabbit immune system by techniques of molecular biology. The inheritance of serum allotypes of two rabbit C kappa isotypes and restriction fragment polymorphisms (RFLPs) of the genes for V kappa, C kappa and the C beta of the Tcr was studied through backcross and F2 matings. The rabbit resembles man in having unlinked C kappa and C beta loci whereas in mice these genes are linked at a distance of approximately 8 centimorgans. We devised a simple and efficient method for determining RFLP types on large numbers of individuals using small samples of peripheral blood or sperm cells. The technique is readily adaptable to any application requiring DNA from small numbers of cells for Southern analyses including pulsed field gel electrophoretic studies. We are studying the low relative expression of the rabbit K2 light chain isotype. A physical map of the duplicated region containing two rabbit kappa light chain genes is being generated. We are studying the atypical expression of VHa2 genes in ali rabbits and the relationship of the mutated chromosome to a VH-CH recombinant chromosome. We are mapping the heavy chain regions of parental and recombinant chromosomes and have discovered new VH-CH recombinant. The present studies place the site of-F-I recombination upstream of JH.